Printed circuit boards are widely used in various kinds of fields, such as electronic devices, communication devices, and calculators. In particular, multi-functionalization, performance enhancement, thinning, and downsizing of small portable devices such as a portable communication device and a notebook computer have been rapidly advanced in recent years. Thus, electronic components such as semiconductor packages which are mounted in such devices have been made thinner and smaller. Accompanied by this situation, there is a need for wiring pattern refining, multilayering, thinning, and performance enhancement (e.g., enhancement of mechanical characteristics) of a printed circuit board which is used to mount these electronic components.
As a printed circuit board has been made thinner, there are concerns such as warpage of a printed circuit board resulting from a difference in coefficient of linear thermal expansion (hereinafter, CTE) between a conductor layer for forming a circuit and an insulating layer or a difference in CTE between a mounting component and an insulating layer. As a method for preventing the warpage, a reduction in CTE of the insulating layer is known to be effective. Accordingly, development of technologies for reducing the CTE of the insulating material constituting the insulating layer has been made. In the case of a multi-layered printed circuit board, electrical connection between layers is achieved by making via holes or the like in an insulating layer. In order to ensure reliability of this electrical connection between layers, it is required to decrease CTE in a board thickness direction.
In order to reduce the CTE of the insulating layer, there is known a method of adding an inorganic filler to a resin composition constituting an insulating layer. Silica is used as the inorganic filler.
For example, PTL 1 proposes an epoxy resin composition for prepregs. The epoxy resin composition essentially contains a predetermined phosphorus compound, a two functional epoxy resin, a polyfunctional epoxy resin, a curing agent, an inorganic filler, and a molybdenum compound. Examples of the inorganic filler include magnesium hydroxide, silica, and talc. A printed circuit board produced from this epoxy resin composition for prepregs is excellent in glass transition temperature (Tg), flame retardancy, heat resistance, and stiffness during heating and is also excellent in hole position accuracy.
Further, PTL 2 proposes a thermosetting resin composition containing a thermosetting resin, silica, and a molybdenum compound where a content of silica is in a range from 20% by volume to 60% by volume inclusive. A printed circuit board produced from this resin composition is excellent in drilling workability, and the insulating layer of the printed circuit board has excellent electrical insulation and low thermal expansibility.
Further, PTL 3 discloses a resin composition containing a reaction product obtained by previously reacting at least a part of hydroxyl groups on polyphenylene ether with epoxy groups on an epoxy compound, a cyanate ester compound, and an organometallic salt. This constitution allows producing a hardened substance that can maintain excellent dielectric characteristics of polyphenylene ether and has an excellent glass transition temperature (Tg).